Cell potential measuring apparatus have been developed to measure the activity or electrical potential generated by activity of nerve cells, other cells, or tissues (for example, Japanese Kokai 8-62209) without inserting glass electrodes or the like into the cells.
Measurement of cell potential by inserting a glass electrode or the like into the cell may damage that cell. Long term measurement of cell potential is quite difficult. It is further difficult to measure plural positions simultaneously; there is a limit to the number of electrodes one can place in a measurement electrode array and it is similarly difficult to adequately determine the position of the sample over measurement electrodes. In contrast, use of a cell potential measuring electrode having plural microelectrodes on a substrate (having a wall for enclosing a region including the microelectrodes), allows cultivation of the cells within the region enclosed by the wall and the simultaneous measurement of the potential of plural positions without damaging those cells.
These cell potential measuring devices measures cell potential against a reference. One such way is discussed with regard to Kokai 8-62209. When 64 microelectrodes are arranged in eight columns and eight rows, theoretically, by using one microelectrode as the reference potential (that is, as a common reference electrode connected to the potential of the culture medium) the cell potential of the other 63 positions can be measured simultaneously by using the remaining 63 microelectrodes.
However, when measuring very low level or micro-potentials such as cell potentials, noise is a problem. Noise level varies significantly depending on the selection of the type and location of the reference electrode. As mentioned above, when using one microelectrode as a reference electrode, simultaneous measurement of potential at 63 positions by using the remaining 63 microelectrodes is impossible because of the high noise level. When the reference electrodes and measuring electrodes correspond one-by-one to each other, the potential may be measured at a very low noise level state; But if 64 microelectrodes are used, for example, corresponding to 32 reference electrodes and 32 measuring electrodes, only 32 positions can be measured simultaneously.
In theory, though, one must limit the number of reference electrodes in order to simultaneously measure the potential at as many positions as possible.
As shown in FIG. 12, eight microelectrodes in one row are used as reference electrodes and seven measuring electrodes each are correlated to each of the reference electrodes, so that the potential can be measured simultaneously at 7.times.8=56 positions. If 56 microelectrodes are used as measuring electrodes, i.e., by using eight microelectrodes in one row as reference electrodes, the loss of measuring sites is about 12% as compared with the case of using all 64 or 63 pieces as measuring electrodes. However, even when seven measuring electrodes are used with one reference electrode, the noise is still quite large. It is quite difficult to detect a small change in cell potential from the noise.
Moreover, as shown in FIG. 12, when placing a segment S of cell or tissue on the plural microelectrodes, the segment S should not be placed on the row of microelectrodes used as reference electrodes. Such a placement requires skill and is difficult because the segment S must be held by tweezers and moved while observing the segment through a microscope. It is extremely difficult to place the segment S so that the eight microelectrodes in one row are completely exposed, while the remaining 56 microelectrodes be completely covered with the segment. If the segment S is placed to completely expose the eight microelectrodes in one row, usually some of the remaining 56 are exposed, and hence the number of positions for simultaneous measurement is decreased.